Recently, lithium ion secondary batteries are used for driving power sources of hybrid electric vehicles and portable electronic devices such as a notebook-size personal computer and a video camcorder.
For example, Patent Literature 1 discloses a lithium ion secondary battery including LiPF6 as a nonaqueous electrolyte (an electrolytic solution) in which the concentration of lithium salt is regulated to 0.4 to 0.8 mol/l.